The invention relates to an optical element which is provided on a substrate and which comprises a light-transmitting layer, the transmission of said light-transmitting layer in the visible region varying in response to a change in ambient light.
The invention further relates to a display device which is provided with an optical element.
Optical elements for varying the transmission of light are used to influence the transmission and/or reflection of (visible) light, for example, of lamps, of rear view mirrors and of sunroofs for cars, or of windows for buildings ("smart windows"). They are also used, for example, in (flat-panel) display devices, such as cathode ray tubes (CRT) and liquid-crystal display devices (LCD, LC-TV and plasma-addressed LCD) to improve the contrast of the image reproduced.
Such layers reduce the intensity of reflected ambient light and the intensity of light originating from an (internal) light source, such as the phosphors in a CRT. Incident ambient light passes through the light-transmitting layer and is reflected at the display screen (for example at the phosphors), whereafter the reflected light passes through the light-transmitting layer again. If the transmission of the light-transmitting layer amounts to T, the intensity of the reflected ambient light decreases by a factor of T.sup.2. Light originating from the internal light source(s), however, passes through the light-transmitting layer only once, so that the intensity of this light decreases only by a factor of T. The combination of these effects leads to an increase of the contrast by a factor of T.sup.-1.
Examples of optical elements for varying the transmission of light include electrochromic elements and photochromic elements.
Oxides of specific transition metals are capable of accepting guest atoms such as hydrogen and alkali-metal atoms. If the oxide forms part of an electrochemical cell, the guest atoms can be accepted and released again in a reversible manner. In general, an electrochromic element comprises a first (transparent, conductive) electrode which is connected to the electrochromic layer, the so-called work electrode, and a second (transparent, conductive) electrode, the so-called counter electrode, which contains a material which serves as a source and as an acceptor for the guest atoms, and an ion-conducting (liquid, polymeric or solid) material, the so-called electrolyte, being present between said two electrodes. The transmission properties of the electrochromic element in the visible region undergo a change when a voltage difference is applied across the electrochromic element.
Such an electrochromic element is disclosed in U.S. Pat. No. 5,060,075 (PHA 40,577), in which the contrast of a luminescent image is increased by providing a front plate of an CRT display device with an electrochromic element, which reduces the reflection of ambient light if the brightness of said ambient light increases. A light sensor, which is provided in the vicinity of the front plate, detects variations in ambient light, and a control circuit, which is electrically connected to the light sensor and the panel, produces, while the display device is in operation, such a control signal that the degree to which the panel passes light decreases as a function of the increase of the intensity of ambient light.
A photochromic element comprises a layer which contains a material whose transmission varies (automatically) as a result of electromagnetic radiation, such as light, which is incident on the layer. A large number of photochromic materials, which can be placed in various categories (for example spiro-pyranes, spiro-oxazines or fulgides) are known from the relevant literature. Such a photochromic element enables, for example, the contrast of a (luminescent) image to be increased by providing a photochromic layer on the display screen of a display device, the local transmission of the layer in the visible region being governed by the radiation which is (locally) incident on the layer. The transmission of the photochromic element depends (preferably) on radiation which impinges on the layer and the wavelength of which is outside the range in which the display device emits light (for example in the so-called UV-A range) and said transmission decreases automatically as the intensity of the incident radiation increases.
Such optical elements often have the disadvantage that their transmission spectrum is not color neutral. As a result, ambient light is reflected in color, which is undesirable.